1. Field of the Invention
The present invention relates to an inverter device for generating a pseudo alternating current by switching a direct current to drive a motor.
2. Description of the Related Art
Conventionally, for example, in an air conditioner for a vehicle, a refrigerant is circulated in a refrigerant circuit by an engine-driven compressor to cool the interior of a compartment. However, attendant upon recent conspicuousness of environmental problems, cars using electric motors, such as electric cars, fuel cell powered cars, and hybrid powered automobiles, have been heartily developed. In such a car, because electric power is supplied from a battery to a main electric motor to run, an air conditioner uses a battery-driven electromotive compressor.
In that case, actually, a direct current from a battery is converted into a three-phase pseudo alternating current by an inverter including a plurality of switching element groups and then applied to a motor of an electromotive compressor to drive. However, if a surplus current flows in the winding of the motor, inconvenience such as burnout may arise. Thus, protection of the motor must be intended. For this reason, in a conventional general air conditioner for a home, the primary current flowing from a power source before entering an inverter is measured to make protection (for example, see Japanese Patent No. 2629664).
Here, in case of making motor protection using the primary current as described above, the secondary current flowing in the motor was calculated from the primary current as follows. That is, the power consumption is represented byW=√{square root over (3)}×Vrs×Ir×cos θ1in case of a three-phase alternating current power source andW=Vdc×Idc in case of a direct current power source, and the power consumption obtained from the secondary side is represented byW=√{square root over (3)}×Vuv×Iu×cos θ2(W: power consumption, Vrs: primary voltage, Ir: primary current, Vuv: secondary voltage, Iu: secondary current, and cos θ: power factor). Therefore, the secondary current Iu is represented byIu=(Vrs×Ir×cos θ1)/(Vuv×cos θ2)orIu=(Vdc×Idc)/(√{square root over (3)}×Vuv×cos θ2).Therefore, for obtaining the secondary current, the values of the power factor, Vrs, Vuv, and Vdc are necessary. If those values include errors, it is difficult to accurately estimate the secondary current.